1. Field of the Invention
The present invention relates to anamorphic objective zoom lenses, and more particularly to a range of different focal length anamorphic objective zoom lenses with zoom ratios that provide imaging over wide to narrow fields of view and provide traditional anamorphic imaging characteristics, and potentially compact diameter by having an arrangement of five lens groups with spherical, anamorphic and spherical powers, in which one of the spherical lens groups following the anamorphic lens group has variable power and an optical stop located inside the last spherical lens group.
2. Description of Prior Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Contemporary anamorphic objective zoom lenses normally have an optical axis and are commonly based on a rear anamorphic lens group or a front anamorphic lens group. Anamorphic objective zoom lenses having a rear anamorphic lens group are typically more commonplace than anamorphic objective zoom lenses having a front anamorphic lens group.
Anamorphic objective zoom lenses having a rear anamorphic lens group have a rear lens group with Y cylinder refractive optical surfaces and a front spherical lens group with an optical stop in the front spherical lens group in the form of a variable aperture diameter iris or diaphragm.
This anamorphic objective zoom lens arrangement produces images having spherical out of focus objects commonly referred to as the bokeh as compared to the oval or elliptically shaped out of focus objects produced by fixed focal length (commonly referred to as prime) anamorphic objective lenses. The oval or elliptically shaped bokeh of out of focus objects are desired by cinematographers because they produce a distinctive artistic look that is different from spherical objective lenses. Another common drawback with this anamorphic objective zoom lens arrangement is that the full aperture may be relatively slow as compared to that of anamorphic prime lenses.
Anamorphic objective zoom lenses having a front anamorphic lens group have a front lens group with X cylinder refractive optical surfaces and a rear spherical lens group with an optical stop in the rear spherical lens group in the form of a variable aperture diameter iris or diaphragm.
This anamorphic objective zoom lens arrangement produces images having oval or elliptically shaped out of focus objects commonly referred to as the bokeh which is desired by cinematographers for the reasons previously given however these lenses normally provide only small zoom ratios of 2× or 3×, where the smaller zoom ratio provides the widest field of view, and they tend be large in diameter with corresponding potentially higher weight and cost. They may also exhibit some breathing when focusing where the breathing is characterized by the field of view or focal length of the lens changing size as the lens is focused from distant to close objects or vice versa. In addition they may also produce a thermally induced focus shift with change of temperature where the focus shift is not constant over wide to narrow fields of view and the image is characterized by having residual astigmatism. Nevertheless the front anamorphic objective zoom lens arrangements produce images having numerous residual optical aberrations and characteristics most of which are desired by cinematographers because they produce an artistic look that is different from spherical objective lenses.
Many of the less desired residual optical aberrations and characteristics of these front and rear anamorphic objective zoom lens arrangements were accepted by cinematographers with film based cameras but with the advent and adoption of electronic sensor based digital cameras some of them have become less acceptable. In particular the amount of residual chromatic aberration has become less tolerable whereas some field curvature combined with some residual astigmatism is still acceptable.
As well as the oval or elliptically shaped bokeh another characteristic that is desired because of the distinctive artistic look produced is the depth of field being different in the vertical azimuth direction of the field versus the horizontal azimuth direction of the field. In the case of an anamorphic objective zoom lens that squeezes the horizontal field of view by substantially two times as compared to the vertical field of view, the depth of field in the horizontal azimuth direction of the field is substantially two times greater than the depth of field in the vertical azimuth direction of the field.
Improving the optical aberrations and characteristics of anamorphic objective zoom lenses of this arrangement may involve increasing optical surface shape complexity and hence manufacturing cost including adding aspherical and free-form shaped optical surfaces.
Thus, to address the artistic need of cinematographers and maximize the imaging potential of both film and digital cameras a compact diameter anamorphic objective zoom lens arrangement that provides a useful zoom range going from wide to narrow fields of view during zooming with a suitable blend of residual optical aberration correction and characteristics needs to be achieved.
Lenses of this type tend to be sensitive to changes in temperature and will go out of focus without some means of compensating for temperature shifts. Accordingly, there is a need for including some means of providing thermal compensation to maintain the focus over the temperature range in which the lens will operate.